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            <h1 id="seo-header">『C++&amp;C语言』Vector shrink 请求容器降低其容量和size匹配 shrink_to_fit();</h1>
            
            
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                <h1 id="『C-C-语言』Vector-shrink-请求容器降低其容量和-size-匹配-shrink-to-fit"><a href="#『C-C-语言』Vector-shrink-请求容器降低其容量和-size-匹配-shrink-to-fit" class="headerlink" title="『C++&amp;C 语言』Vector shrink 请求容器降低其容量和 size 匹配 shrink_to_fit();"></a>『C++&amp;C 语言』Vector shrink 请求容器降低其容量和 size 匹配 shrink_to_fit();</h1><h5 id="一、先从-size-和-capacity-说起"><a href="#一、先从-size-和-capacity-说起" class="headerlink" title="一、先从 size 和 capacity 说起"></a>一、先从 size 和 capacity 说起</h5><p><strong>resize()，设置大小（size）;<br>reserve()，设置容量（capacity）;</strong><br>size()是分配容器的内存大小，而 capacity()只是设置容器容量大小，但并没有真正分配内存。 打个比方：买了一个新房子，新房子里可以放 3 张床 reserve(3)，这是说房子的容量是最多放 3 张床，但是屋里并不是有三张床，二 resize(3),房里安装了 3 张床，此时房里的床可以使用了。<br><strong>reserve</strong>为容器预留足够的空间，避免不必要的重复分配，分配空间大于等于函数的参数，影响 capacity。但 reserve 的功能确实蹩脚，只能用 reserve 是的 capacity 变得比之前大。<br><strong>resize</strong>调整容器中有效数据区域的尺寸，如果尺寸变小，原来数据多余的截掉。若尺寸变大，不够的数据用该函数第二个参数填充,影响 size。<br>由于 vector 是顺序容器，在内存中分配了一块连续的存储空间。为了保证动态添加元素的高效率，因此必须预先为 vector 分配一段空间，这个空间就是 capacity。<br><strong>而容器中元素的个数就是 size()，在容器中，capacity 总是大于等于 size；</strong><br>当 vector 数组插入数据量过大时，其 capacity，会变得很大，且清空 vector 容器后，还会保留原分配的容量 capacity。系统不会自动收回空间吗？<strong>真的不会!!!!!!!！</strong><br><strong>我们一点一点写程序把 risize()跟 reserve()弄那个明白。</strong></p>
<figure class="highlight lisp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><code class="hljs lisp">vector&lt;int&gt; <span class="hljs-literal">t</span><span class="hljs-comment">;</span><br>for(<span class="hljs-name">int</span> i=0<span class="hljs-comment">;i&lt;1000;i++)</span><br>   &#123;<br>       <span class="hljs-literal">t</span>.push_back(<span class="hljs-name">i</span>)<span class="hljs-comment">;</span><br>   &#125;<br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.resize(<span class="hljs-number">5</span>)<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.reserve(<span class="hljs-number">1</span>)<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.resize(<span class="hljs-number">20</span>)<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.reserve(<span class="hljs-number">2000</span>)<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.resize(<span class="hljs-number">10000</span>)<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br></code></pre></td></tr></table></figure>

<p><strong>运行结果</strong><br><img src="https://img-blog.csdnimg.cn/20190314162733973.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzYyNzExOA==,size_16,color_FFFFFF,t_70" srcset="/img/loading.gif" lazyload alt="在这里插入图片描述"><br>我们可以看出当 Vector 内只有五个元素时其分配空间还是 1024，而 reserve 却不能做出任何反应，蹩脚，但是 reserve 能让容器空间变大，其实 vector 既然是容器他就会自动分配更多空间，所以 reserve 差评，这不是重点，重点是怎么将 vector 多分配出来的空间收回。有同学要说了 clear()。</p>
<figure class="highlight lisp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><code class="hljs lisp">vector&lt;int&gt; <span class="hljs-literal">t</span><span class="hljs-comment">;</span><br>for(<span class="hljs-name">int</span> i=0<span class="hljs-comment">;i&lt;1000;i++)</span><br>   &#123;<br>       <span class="hljs-literal">t</span>.push_back(<span class="hljs-name">i</span>)<span class="hljs-comment">;</span><br>   &#125;<br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.clear()<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br></code></pre></td></tr></table></figure>

<p><strong>运行结果</strong><br><img src="https://img-blog.csdnimg.cn/20190314163134510.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzYyNzExOA==,size_16,color_FFFFFF,t_70" srcset="/img/loading.gif" lazyload alt="在这里插入图片描述"><br>然而 clear 只是将容器内的元素清空了，对于分配的 capacity，却没有作用。在这里有几种方法实现降低容量 ，但是其原理相同。</p>
<figure class="highlight lisp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><code class="hljs lisp">vector&lt;int&gt; <span class="hljs-literal">t</span><span class="hljs-comment">;</span><br>   for(<span class="hljs-name">int</span> i=0<span class="hljs-comment">;i&lt;1000;i++)</span><br>   &#123;<br>       <span class="hljs-literal">t</span>.push_back(<span class="hljs-name">i</span>)<span class="hljs-comment">;</span><br>   &#125;<br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.clear()<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.shrink_to_fit()<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br></code></pre></td></tr></table></figure>

<p>运行结果<br><img src="https://img-blog.csdnimg.cn/20190314163431923.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzYyNzExOA==,size_16,color_FFFFFF,t_70" srcset="/img/loading.gif" lazyload alt="在这里插入图片描述"><br>当时我比较苦恼时大佬给我了两个方法，上面那个，还有一个底层的写法如下。</p>
<figure class="highlight lisp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><code class="hljs lisp">vector&lt;int&gt; <span class="hljs-literal">t</span><span class="hljs-comment">;</span><br>   for(<span class="hljs-name">int</span> i=0<span class="hljs-comment">;i&lt;1000;i++)</span><br>   &#123;<br>       <span class="hljs-literal">t</span>.push_back(<span class="hljs-name">i</span>)<span class="hljs-comment">;</span><br>   &#125;<br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   <span class="hljs-literal">t</span>.clear()<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   auto newt=t<span class="hljs-comment">;</span><br>   swap(<span class="hljs-name">newt</span>,<span class="hljs-literal">t</span>)<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>   cout&lt;&lt;newt.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>   cout&lt;&lt;newt.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br></code></pre></td></tr></table></figure>

<p><img src="https://img-blog.csdnimg.cn/20190314164146857.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzYyNzExOA==,size_16,color_FFFFFF,t_70" srcset="/img/loading.gif" lazyload alt="在这里插入图片描述"><br>显然 t 容器已经被降低容量，但是其容量降低的代价时 newt 的容量变大。所以这种方法不可取。还有第三种方法。</p>
<figure class="highlight lisp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><code class="hljs lisp">vector&lt;int&gt; <span class="hljs-literal">t</span><span class="hljs-comment">;</span><br>  for(<span class="hljs-name">int</span> i=0<span class="hljs-comment">;i&lt;1000;i++)</span><br>  &#123;<br>      <span class="hljs-literal">t</span>.push_back(<span class="hljs-name">i</span>)<span class="hljs-comment">;</span><br>  &#125;<br>  cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>  cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>  <span class="hljs-literal">t</span>.clear()<span class="hljs-comment">;</span><br>  cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>  cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br>  vector&lt;int&gt;(<span class="hljs-name">t</span>).swap(<span class="hljs-name">t</span>)<span class="hljs-comment">;</span><br>  cout&lt;&lt;t.size()&lt;&lt;&#x27; &#x27;<span class="hljs-comment">;</span><br>  cout&lt;&lt;t.capacity()&lt;&lt;endl<span class="hljs-comment">;</span><br></code></pre></td></tr></table></figure>

<p><img src="https://img-blog.csdnimg.cn/20190314164822483.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzYyNzExOA==,size_16,color_FFFFFF,t_70" srcset="/img/loading.gif" lazyload alt="在这里插入图片描述"><br>以上就是对于 vector 的 capacity 的探究，当数据量较少时，多分配的 capacity 可以忽略，但是当数据量很大之后，就不能忽略了，所以当你 clear 之后记着 shrink 呀。</p>

                
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